Flexography is a method of printing that is commonly used for high-volume runs. Flexography is employed for printing on a variety of substrates such as paper, paperboard stock, corrugated board, films, foils and laminates. Newspapers and grocery bags are prominent examples. Coarse surfaces and stretch films can be economically printed only by means of flexography.
Flexographic printing plates are relief plates with image elements raised above open areas. Generally, the plate is somewhat soft, and flexible enough to wrap around a printing cylinder, and durable enough to print over a million copies. Such plates offer a number of advantages to the printer, based chiefly on their durability and the ease with which they can be made. A typical flexographic printing plate as delivered by its manufacturer is a multilayered article made of, in order, a backing or support layer; one or more unexposed photocurable layers; optionally a protective layer or slip film; and often, a protective cover sheet.
The support (or backing) layer lends support to the plate. The support layer can be formed from a transparent or opaque material such as paper, cellulose film, plastic, or metal. Preferred materials include sheets made from synthetic polymeric materials such as polyesters, polystyrene, polyolefin, polyamides, and the like, including polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polybutylene terephthalate (PBT). The support may be in sheet form or in cylindrical form, such as a sleeve. The sleeve may be formed from single layer or multiple layers of flexible material. Flexible sleeves made of polymeric films are preferred, as they typically are transparent to ultraviolet radiation and thereby accommodate backflash exposure for building a floor in the cylindrical printing element. One widely used support layer is a flexible film of polyethylene terephthalate.
The photocurable layer(s) can include any of the known photopolymers, monomers, initiators, reactive or non-reactive diluents, fillers, processing aids, UV absorbers and dyes. As used herein, the term “photocurable” refers to a composition which undergoes polymerization, cross-linking, or any other curing or hardening reaction in response to actinic radiation with the result that the unexposed portions of the material can be selectively separated and removed from the exposed (cured) portions to form a three-dimensional relief pattern of cured material. Exemplary photocurable materials are disclosed in European Patent Application Nos. 0 456 336 A2 and 0 640 878 A1 to Goss, et al., British Patent No. 1,366,769, U.S. Pat. No. 5,223,375 to Berrier, et al., U.S. Pat. No. 3,867,153 to MacLahan, U.S. Pat. No. 4,264,705 to Allen, U.S. Pat. Nos. 4,323,636, 4,323,637, 4,369,246, and 4,423,135 all to Chen, et al., U.S. Pat. No. 3,265,765 to Holden, et al., U.S. Pat. No. 4,320,188 to Heinz, et al., U.S. Pat. No. 4,427,759 to Gruetzmacher, et al., U.S. Pat. No. 4,622,088 to Min, and U.S. Pat. No. 5,135,827 to Bohm, et al., the subject matter of each of which is herein incorporated by reference in its entirety. More than one photocurable layer may also be used. The photocurable layer(s) may be applied directly on the support. In the alternative, the photocurable layer(s) may be applied on top of an adhesion layer and/or resilient under layer.
Photocurable materials generally cross-link (cure) and harden through radical polymerization in at least some actinic wavelength region. As used herein, “actinic radiation” is radiation that is capable of polymerizing, crosslinking or curing the photocurable layer. Actinic radiation includes, for example, amplified (e.g., laser) and non-amplified light, particularly in the UV and violet wavelength regions.
The slip film is a thin layer, which protects the photopolymer from dust and increases its ease of handling. In a conventional (“analog”) plate making process, the slip film is transparent to UV light, and the printer peels the cover sheet off the printing plate blank, and places a negative on top of the slip film layer. The plate and negative are then subjected to flood-exposure by UV light through the negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. In the alternative, a negative may be placed directly on the at least one photocurable layer.
In a “digital” or “direct to plate” process, a laser is guided by an image stored in an electronic data file, and is used to create an in situ negative in a digital (i.e., laser ablatable) masking layer, which is generally a slip film which has been modified to include a radiation opaque material. Portions of the laser ablatable layer are then ablated by exposing the masking layer to laser radiation at a selected wavelength and power of the laser. Thereafter, the at least one photocurable layer with the in situ negative thereon, is subjected to flood-exposure by UV light through the in situ negative. The areas exposed to the light cure, or harden, and the unexposed areas are removed (developed) to create the relief image on the printing plate. Selective exposure to the source of actinic radiation can be achieved using either the analog or digital method. Examples of laser ablatable layers are disclosed, for example, in U.S. Pat. No. 5,925,500 to Yang, et al., and U.S. Pat. Nos. 5,262,275 and 6,238,837 to Fan, the subject matter of each of which is herein incorporated by reference in its entirety.
Processing steps for forming relief image printing elements typically include the following:                1) Image generation, which may be mask ablation for digital “computer to plate” printing plates or negative production for conventional analog plates;        2) Face exposure through the mask (or negative) to selectively crosslink and cure portions of the photocurable layer not covered by the mask, thereby creating the relief image;        3) Back exposure to create a floor layer in the photocurable layer and establish the depth of relief. It is preferred to face expose the plate before flipping it for back exposure. Doing the back exposure first may result in damaging the black mask during the plate handling, thus contributing to image degradation. Some exposing systems can also run both exposure systems simultaneously, which also preserves the image integrity;        4) Development to remove unexposed photopolymer by solvent (including water) or dry “thermal” development; and        5) If necessary, post exposure and detackification.        
Removable coversheets may be provided to protect the photocurable printing element from damage during transport and handling. Useful cover sheets include flexible polymeric films, e.g., polystyrene, polyethylene, polypropylene, polycarbonate, fluoropolymers, polyamide or polyesters. Polyesters, especially polyethylene terephthalate, are commonly used.
Prior to processing the printing elements, the coversheet(s) are removed and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer solvent (or thermal development) removes the unexposed areas of the photopolymerizable layer, leaving a printing relief that can be used for flexographic printing.
As used herein “back exposure” refers to a blanket exposure to actinic radiation of the photopolymerizable layer on the side opposite that which does, or ultimately will, bear the relief. This step is typically accomplished through a transparent support layer and is used to create a shallow layer of photocured material, i.e., the “floor,” on the support side of the photocurable layer. The purpose of the floor is generally to sensitize the photocurable layer and to establish the depth of relief.
Prior to the brief back exposure step (i.e., brief as compared to the imagewise exposure step), an imagewise exposure is performed utilizing a digitally-imaged mask or a photographic negative mask, which is in contact with the photocurable layer and through which actinic radiation is directed.
The type of radiation used is dependent in part on the type of photoinitiator in the photopolymerizable layer. The digitally-imaged mask or photographic negative prevents the material beneath from being exposed to the actinic radiation and hence those areas covered by the mask do not polymerize, while the areas not covered by the mask are exposed to actinic radiation and polymerize. Any conventional sources of actinic radiation can be used for this exposure step. Examples of suitable visible and UV sources include carbon arcs, mercury-vapor arcs, fluorescent lamps, electron flash units, electron beam units, photographic flood lamps, and, more recently, light emitting diodes (LEDs), which emit UV light.
The use of ultraviolet mercury arc lamps that emit ultraviolet light suitable to cure photocurable layers is well known. Ultraviolet arc lamps emit light by using an electric arc to excite mercury that resides inside an inert gas (e.g., argon) environment to generate ultraviolet light which effectuates curing. Alternatively, microwave energy can also be used to excite mercury lamps in an inert gas medium to generate the ultraviolet light. However, the use of ultraviolet mercury lamps as a radiation source suffers from several disadvantages including environmental concerns from mercury and the generation of ozone as a by-product. Further, mercury lamps typically have lower energy conversion ratio, require warm-up time, generate heat during operation, and consume a large amount of energy when compared with LEDs. In addition, mercury lamps are characterized by a broad spectral output, in addition to the UV radiation, much of which is not useful for curing and can damage substrates and presents hazards to personnel.
LEDs are semiconductor devices which use the phenomenon of electroluminescence to generate light. LEDs consist of a semiconducting material doped with impurities to create a p-n junction capable of emitting light as positive holes join with negative electrons when voltage is applied. The wavelength of emitted light is determined by the materials used in the active region of the semiconductor. Typical materials used in semiconductors of LEDs include, for example, elements from Groups (III) and (V) of the periodic table. These semiconductors are referred to as III-V semiconductors and include, for example, GaAs, GaP, GaAsP, AlGaAs, InGaAsP, AlGaInP and InGaN semiconductors. The choice of materials is based on multiple factors including desired wavelength of emission, performance parameters and cost.
It is possible to create LEDs that emit light anywhere from a low of about 100 nm to a high of about 900 nm. Currently, known LED UV light sources emit light at wavelengths between about 300 and about 475 nm, with 365 nm, 390 nm and 395 nm being common peak spectral outputs. When using LED lamps for curing photocurable compositions, the photoinitiator is the coating composition is selected to be responsive to the wavelength of light emitted by the LED lamp.
LED offer several advantages over mercury lamps in curing applications. For example, LEDs do not use mercury to generate UV light and are typically less bulky than mercury UV arc lamps. In addition, LEDs are instant on/off sources requiring no warm-up time, which contributes to LED lamps' low energy consumption. LEDs also generate much less heat, with higher energy conversion efficiency, have longer lamp lifetimes, and are essentially monochromatic emitting a desired wavelength of light which is governed by the choice of semiconductor materials employed in the LED.
In the analog flexographic printing element world, higher UV doses are often required to hold finer highlight dots. If a desired dot level would not hold, then the operator would increase the dose by increasing the amount of time that the plate was exposed. This would work for most plate formulation types. However, the downside is that as exposure times are increased to hold finer dots, reverses often start to close up. This is commonly referred to as the plate's exposure latitude.” Therefore, one can increase the dot sensitivity only to the point where the reverses stay open. Thus, there remains a need in the art for an improved photocurable composition that can be imagewise exposed to actinic radiation from an LED source at a chosen wavelength while exhibiting a good exposure latitude.